With the advent of genetic engineering, the large-scale availability of many bioactive peptides and proteins has been achieved. However, the administration of these recombinantly produced peptides and proteins presents a unique set of problems. In many cases the maintenance of the biological effect of these proteins requires long-term administration. Daily administration of these agents in aqueous vehicles is inconvenient and costly; sustained or prolonged release is preferred. In addition, proteins are highly unstable in an aqueous environment most suitable for administration.
For instance, bovine somatotropin (bSt) is being produced by recombinant DNA technology. It is well recognized that lactating dairy cows administered recombinant bSt (rbSt) produce more milk compared with controls. Administration of rbSt in a sustained release vehicle will allow less frequent dosing which would be expected to result in a savings of time and labor to dairymen.
Biocompatible oils have long been used as vehicles to provide sustained delivery of many drugs, including proteins and specifically somatotropins. Oleaginous vehicles also have the practical advantage of providing a low water content environment to the suspended, dehydrated protein, thus providing sufficient stability for long term storage of a ready-to-inject product.
An alternative approach to stabilization involves the use of a non-aqueous, water miscible vehicle. Numerous such vehicles are known, e.g., propylene glycol, glycerin, dimethyl sulfoxide (DMSO), the polyethylene glycols, triacetin, n-methylpyrrolidone, 2-pyrrolidone, and N,N-dimethylacetamide. However, at protein concentrations needed for an extended release product, these vehicles typically either do not form a suspension or, when mixture is possible, form a suspension not suitable for a commercial product. I have found an exception to these formulation difficulties in glyceryl triacetate (triacetin) and in the polyethylene glycols (PEG).
Triacetin is a well known article of commerce which is used primarily in industrial applications. A number of pharmaceutical uses have been reported, e.g., as a solvent for preparing stable solution formulations of prostaglandin-like compounds (U.S. Pat. Nos. 3,966,962, 4,301,175, and 4,680,312), as an antifungal agent (U.S. Pat. No. 3,070,497), as a solvent in tetracycline solutions (U.S. Pat. No. 3,219,529), as a solvent in various topical formulations (GB 2150830, JP 63130541, and JP 59044311 (as a plasticizer in polymeric drug delivery systems); U.S. Pat. No. 4,857,313 (as a co-polymer for transdermal delivery systems), and as a dispersing agent in the food industry for hydrophilic colloids such as gelatin (GB 1185340). Triacetin is included in the GRAS (generally regarded as safe) list compiled by the U.S. Food and Drug Administration. The Merck Index, 10th Edition, reports triacetin is soluble in water to the extent of about one part triacetin to 14 parts water.
The polyethylene glycols are well known articles of commerce which have been put to a variety of applications. For instance, the PEGs are used as plasticizers in other polymeric systems, the high molecular weight PEGs as hydrophilic excipients in compressed tablets for oral drug delivery, and within osmotic minipumps. They are commonly used at low concentrations as dispersing agents in aqueous suspensions and as cosolvents with water or ethanol. For instance, U.S. Pat. No. 4,041,155 discloses a composition of an aqueous formulation of 80% PEG 400 and somatostatin which reportedly sustained release of the active agent for 4 hours. Finally, European Patent Application 0 140 255 discloses a sustained release composition of interferon and a biodegradable carrier which is dispersed in a viscous solvent, e.g. polyethylene glycol. To my knowledge, there are no published reports of the use of a polyethylene glycol suspension for sustained release of a bioactive protein.
Polyethylene glycols (.alpha.-hydro-.omega.-hydroxypoly(oxy-1,2-ethanediyl) have the general formula H(OCH.sub.2 CH.sub.2).sub.n OH where n.gtoreq.4 and are included in the GRAS (generally regarded as safe) list compiled by the U.S. Food and Drug Administration.
The ease and rapidity which somatotropin and growth hormone releasing factor are suspended either in triacetin and in polyethylene glycol is entirely unexpected in view of the failure of numerous non-aqueous, water-miscible solvents as suspension vehicles. I have found that these non-aqueous, water-miscible organic solvents provide unexpectedly and surprisingly prolonged release of proteins suspended therein.